Sensors are well known in the art. When formed in a semiconductor material such as silicon or germanium such sensors may be provided as mechanical structures, for example as a MEMS arrangement, or electro-magnetic (EM) radiation sensors such as infra-red (IR) sensors. By using materials such as silicon it is possible to form the sensor in one or more layers of the wafer from etching and other semiconductor processing techniques so as to result in a desired configuration. Due to the delicate nature of the sensors and their sensitivity to the surrounding environment it is known to provide a protective cap over the sensor, the cap serving to isolate the environment of the sensor from the ambient environment where the sensor is operable.
Current infrared absorption gas sensors frequently use a discrete thermal sensor, e.g. a thermopile, with an external thin film filter which provides a received energy wavelength response that is tuned to the gas absorption band of interest. While generally effective, this provides a solution which requires more assembly operations and therefore cost, resulting in a more expensive device. Creation of a low cost means of providing sensor and filtering function, possibly together with signal processing electronics all in one device has been difficult as the thin film filters are specialised and difficult to manufacture with the standard IC processing equipments and materials.